Seasonal metabolic changes in a year-round reproductively active subtropical tree-frog (Hypsiboas prasinus)

Although seasonal metabolic variation in ectothermic tetrapods has been investigated primarily in the context of species showing some level of metabolic depression during winter, but several species of anurans maintain their activity patterns throughout the year in tropical and subtropical areas. The tree-frog Hypsiboas prasinus occurs in the subtropical Atlantic Forest and remains reproductively active during winter, at temperatures below 10 degrees C. We compared males calling in summer and winter, and found that males of H. prasinus exhibit seasonal adjustments in metabolic and morphometric variables. Individuals calling during winter were larger and showed higher resting metabolic rates than those calling during summer. Calling rates were not affected by season. Winter animals showed lower liver and heart activity level of citrate synthase (CS), partially compensated by larger liver mass. Winter individuals also showed higher activity of pyruvate kinase (PK) and lower activity of CS in trunk muscles, and higher activity of CS in leg muscles. Winter metabolic adjustments seem to be achieved by both compensatory mechanisms to the lower environmental temperature and a seasonally oriented aerobic depression of several organs. The impact of seasonal metabolic changes on calling performance and the capacity of subtropical anurans for metabolic thermal acclimatization are also discussed.     

Winter climate change affects growing‐season soil microbial biomass and activity in northern hardwood forests

Understanding the responses of terrestrial ecosystems to global change remains a major challenge of ecological research. We exploited a natural elevation gradient in a northern hardwood forest to determine how reductions in snow accumulation, expected with climate change, directly affect dynamics of soil winter frost, and indirectly soil microbial biomass and activity during the growing season. Soils from lower elevation plots, which accumulated less snow and experienced more soil temperature variability during the winter (and likely more freeze/thaw events), had less extractable inorganic nitrogen (N), lower rates of microbial N production via potential net N mineralization and nitrification, and higher potential microbial respiration during the growing season. Potential nitrate production rates during the growing season were particularly sensitive to changes in winter snow pack accumulation and winter soil temperature variability, especially in spring. Effects of elevation and winter conditions on N transformation rates differed from those on potential microbial respiration, suggesting that N‐related processes might respond differently to winter climate change in northern hardwood forests than C‐related processes.     

Cold Hardening of Spring and Winter Wheat and Rape Results in Differential Effects on Growth,Carbon Metabolism,and Carbohydrate Content

The effect of long-term (months) exposure to low temperature (5[deg]C) on growth, photosynthesis, and carbon metabolism was studied in spring and winter cultivars of wheat (Triticum aestivum) and rape (Brassica napus). Cold-grown winter rape and winter wheat maintained higher net assimilation rates and higher in situ CO2 exchange rates than the respective cold-grown spring cultivars. In particular, the relative growth rate of spring rape declined over time at low temperature, and this was associated with a 92% loss in in situ CO2 exchange rates. Associated with the high photosynthetic rates of cold-grown winter cultivars was a 2-fold increase per unit of protein in both stromal and cytosolic fructose-1,6-bisphosphatase activity and a 1.5- to 2-fold increase in sucrose-phosphate synthase activity. Neither spring cultivar increased enzyme activity on a per unit of protein basis. We suggest that the recovery of photosynthetic capacity at low temperature and the regulation of enzymatic activity represent acclimation in winter cultivars. This allow these overwintering herbaceous annuals to maximize the production of sugars with possible cryoprotective function and to accumulate sufficient carbohydrate storage reserve to support basal metabolism and regrowth in the spring.     

Size and temperature dependent foraging capacities and metabolism: consequences for winter starvation mortality in fish

The foraging related capacities, energy requirements and the ability of individuals to withstand starvation are strongly dependent on body size and temperature. In this study, we estimated size-dependent foraging rates and critical resource density (CRD) in small Arctic char ( Salvelinus alpinus ) under winter conditions and compared these with previous observations under summer conditions. We investigated if starvation mortality is size-dependent in the laboratory, and we assessed the potential for winter growth and the occurrence and size dependency of winter mortality both in a large scale pond experiment and in natural lakes. The efficiency of foraging on macroinvertebrates increased with size but was lower at 4 than 12°C, still CRD was lower at 4 than 12°C as metabolic rates decreased faster than foraging efficiency with temperature. When starved, small char died before large and at rates which suggest that YOY char need to feed during winter to avoid starvation. Results from both our pond experiment and field study indicate that survival of YOY char over winter is high, despite severe winter conditions, because YOY char are able to feed and grow during winter. In seasonal environments with declining resources, the size scaling and temperature dependency of foraging and metabolic demands may provide conditions which can favour either small or large individuals. This size advantage dichotomy relates to that larger individual's by having a higher CRD are more likely to start starving, but once resource levels are below CRD for all size classes, small individuals starve to death at a higher rate. Negative size-dependent winter mortality from starvation is suggested to be more pronounced in species that are not adapted to feed at low temperatures and in species feeding on zooplankton, since zooplankton abundance, in contrast to macroinvertebrate abundance, is generally low during winter.     

Winter Climate Limits Subantarctic Low Forest Growth and Establishment

Campbell Island, an isolated island 600 km south of New Zealand mainland (52°S, 169°E) is oceanic (Conrad Index of Continentality  = −5) with small differences between mean summer and winter temperatures. Previous work established the unexpected result that a mean annual climate warming of c. 0.6°C since the 1940''s has not led to upward movement of the forest limit. Here we explore the relative importance of summer and winter climatic conditions on growth and age-class structure of the treeline forming species, Dracophyllum longifolium and Dracophyllum scoparium over the second half of the 20^th century. The relationship between climate and growth and establishment were evaluated using standard dendroecological methods and local climate data from a meteorological station on the island. Growth and establishment were correlated against climate variables and further evaluated within hierarchical regression models to take into account the effect of plot level variables. Winter climatic conditions exerted a greater effect on growth and establishment than summer climatic conditions. Establishment is maximized under warm (mean winter temperatures >7 °C), dry winters (total winter precipitation <400 mm). Growth, on the other hand, is adversely affected by wide winter temperature ranges and increased rainfall. The contrasting effect of winter warmth on growth and establishment suggests that winter temperature affects growth and establishment through differing mechanisms. We propose that milder winters enhance survival of seedlings and, therefore, recruitment, but increases metabolic stress on established plants, resulting in lower growth rates. Future winter warming may therefore have complex effects on plant growth and establishment globally.     

Seasonal variations in basal metabolic rate, lower critical temperature and responses to temporary starvation in the arctic fox (Alopex lagopus) from Svalbard

Metabolic rates of four resting, post-absorptive male adult summer- and winter-adapted captive arctic foxes (Alopex lagopus) were recorded. Basal metabolic rates (BMR) varied seasonally with a 36% increase from winter to summer, while body mass was reduced by 17% in the same period. The lower critical temperature (T _1c) of the winter-adapted arctic fox was estimated to −7°C, whereas T _lc during summer was 5°C. The similarity of these values, which are much higher than hitherto assumed (e.g. Scholander et al. 1950b), is mainly due to a significantly (P<0.05) lower BMR in winter than in summer. Body core (stomach) temperature was stable, even at ambient temperatures as low as −45°C, but showed a significant (P<0.05) seasonal variation, being lower in winter (39.3±0.33°C) than in summer (39.8±0.16°C). The thermal conductivity of arctic fox fur was the same during both seasons, whereas the thermal conductance in winter was lower than in summer. This was reflected in an increase in fur thickness of 140% from summer to winter, and in a reduced metabolic response to ambient temperatures below T _lc in winter. Another four arctic foxes were exposed to three periods of forced starvation, each lasting 8 days during winter, when body mass is in decline. No significant reduction in mass specific BMR was observed during the exposure to starvation, and respiratory quotient was unchanged at 0.73±0.02 during the first 5 days, but dropped significantly (P<0.05) to 0.69±0.03 at day 7. Locomotor activity and body core (intraperitoneal) temperature was unaltered throughout the starvation period, but body mass was reduced by 18.5±2.1% during these periods. Upon re-feeding, locomotor activity was significantly (P<0.05) reduced for about 6 days. Energy intake was almost doubled, but stabilised at normal levels after 11 days. Body mass increased, but not to the level before the starvation episodes. Instead, body mass increased until it reached the reduced body mass of ad libitum fed control animals. This indicates that body mass in the arctic fox is regulated according to a seasonally changing set point.     

Seasonal temperature compensation in the horse mussel, Modiolus modiolus: metabolic enzymes, oxidative stress and heat shock proteins

Seasonal collections of the subtidal horse mussel, Modiolus modiolus, from a depth of 10 m were made at the Isles of Shoals, New Hampshire to assess changes in overall energetic demand, measured as respiration, the maximal activities of rate-limiting enzymes of intermediate metabolism, level of oxidative stress, and the expression of heat shock proteins (HSP). Weighted respiration rates of mussels from winter collections were significantly lower than summer rates but decreased by less than 20%. Specific activities of several rate-limiting enzymes were measured in mussels from the summer and winter collections at the temperature of collection and the reciprocal seasonal temperature (15 and 5 degrees C). Comparisons of these enzyme activities and the protein concentrations of hexokinase and citrate synthase show that a quantitative strategy is used to acclimatize to winter temperatures by these rate-limiting enzymes of intermediate metabolism. The activities and protein concentrations of the antioxidant enzyme, Cu/Zn superoxide dismutase (SOD) is seasonally indistinguishable while the concentration of HSP 70 was greater in winter than in summer samples. These results show that mussels seasonally compensate for decreases in temperature by increasing the concentration of rate-limiting metabolic enzymes while maintaining the same level of antioxidant protection in summer and winter consistent with high aerobic metabolism in both winter and summer. Lastly, the significantly greater concentrations of HSP70 in winter samples suggests that protein chaperone functions must be maintained while other seasonal adjustments to cold temperatures are occurring.     

Differential effects of adrenergic blockade on seasonal changes in core temperatures and thermal conductances of deer mice maintained in thermal neutral environments

1. Resting, daytime, thermal conductances and metabolic rates of mice conditioned to winter (10:14LD) and summer (14:10LD) photoperiods were reduced by social huddling; huddling resulted in group size related elevations in core temperature during summer, but not with winter light-dark cycle exposures. 2. Core temperatures of resting, solitary winter animals were lower than those of summer; both summer and winter animals' core temperatures were further reduced by increased thermal conductance resulting from (phentolamine) alpha receptor blockade. 3. Social huddling reduction of the heat loss from phentolamine treatment was more effective for winter (10:14LD) animals. 4. While phentolamine treatment resulted in increased thermal conductance and lower core temperatures of the mice, propranalol treatment resulted in lower core temperatures and resting metabolic rates, with a resulting decrease in thermal conductance. 5. Since adrenergic blockade was less dose-effective on winter animals, we reasoned that winter animals display higher levels of endogenous adrenergic capacity than summer animals and that lower winter thermoregulatory set points provide for energy conservation with enhanced capacity for meeting cold challenge.     

Lower body mass and higher metabolic rate enhance winter survival in root voles,Microtus oeconomus

Although the biological significance of individual variation in physiological traits is widely recognized, studies of their association with fitness in wild populations are surprisingly scarce. We investigated the effect of individual phenotypic variation in body mass, resting (RMR) and peak metabolic rates (PMR) on mortality of the root vole Microtus oeconomus. Body mass and metabolic rates varied significantly among consecutive years and were also age dependent, as individuals born in late summer and autumn were characterized by significantly lower body mass and metabolic rates than animals born earlier. At the beginning of winter voles born in spring and early summer exhibited reduced body mass and metabolic rates, whereas animals born later maintained lower body mass and RMR, which may be interpreted as phenotypic plasticity enhancing the probability of survival. Body mass had no significant effect on vole survival during summer. In contrast, smaller individuals were characterized by lower mortality during early winter, whereas higher body mass was positively associated with survival later in the season. High body‐mass‐corrected RMR positively affected survival in both summer and winter. The effect of PMR was apparent only during winter, though its direction (and correlation with RMR) varied among years. Deep snow cover negatively affected the survival of voles in both early and late winter. Ambient temperature was positively associated with winter survival, except for late winter, when rising temperature caused flooding of vole habitat. We conclude that the lack of consistency in the directionality and strength of the effects of body mass and metabolic rates on winter survival does not undermine their importance, but rather demonstrates the ability of individuals to adjust metabolic rate to changing environmental conditions. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 297–309.     

Seasonal acclimatization in metabolic rate of the fan-fingered gecko,Ptyodactylus hasselquistii (Reptilia: Gekkonidae)

The resting metabolic rate of the fan-fingered gecko Ptyodactylus hasselquistii of various body masses was determined in relation to ambient temperatures ranging from 20 to 35°C during winter and summer acclimatization. Oxygen consumption (ml g⁻¹ h⁻¹) decreased with increasing mass at each temperature. The intraspecific exponents of body mass in relation to metabolic rate ranged from 0.62 to 0.79. Winter-acclimatized geckos had significantly lower metabolic rates than summer-acclimatized geckos at different temperatures, especially at low temperature (20°C). The pattern of acclimatization exhibited by P. hasselquistii may conserve energy during inactivity in winter and make activity more easily achieved during active seasons.     

Growth Substances and Dormancy in Ceratophyllum demersum

Ceratophyllum demersum L. occurs in winter in the dormant form, in summer in the vegetative form. Factors that affect growth and dormancy in Ceratophyllum were studied. After several weeks of severe winter conditions the plants changed from dormant to quiescent state. Under natural conditions Ceratophyllum plants remain quiescent for several months, due to unfavourable growth conditions. Experimentally the dormant could also be broken by high and low temperature treatments (shocks), and most effectively by addition of GA, An attempt to induce dormancy in full grown plants by the addition of ABA under extreme summer or winter conditions proved unsuccessful. The IAA and ABA contents in the plants were measured during the year. In winter the concentration of ABA was high and that of IAA low, whereas in summer the IAA concentration increased and that of ABA was variable. IAA only slightly antagonized the inhibition of growth by ABA. Both the growth regulators were readily taken up from the culture medium, as was confirmed by a study with the radioactive labelled compounds. The uptake rate of IAA was significantly higher than that of ABA. being 762 μg and 3.26, μg per plant in 24 h, respectively. GA, was found to have a strong antagonistic effect on the ABA induced growth inhibition. The total GA activity in dormant and quiescent plants was similar, in full grown plants it was much lower. In the dormant state a large part of GA was in a bound form, whereas during quiescence relatively more GA occurred in a free state in the plants.     

Maximum and effective PSII yields in the cortex of the main stem of young <Emphasis Type="Italic">Prunus cerasus</Emphasis> trees: effects of seasons and exposure

Seasonal changes in chlorophyll fluorescence parameters of corticular chlorenchyma in the main trunk of Prunus cerasus were followed in the field under ambient temperature and light conditions during bright days. Concomitantly, measurements of periderm light transmittance also allowed the calculation of linear electron transport rates along PSII. Pre-dawn PSII photochemical efficiency was high during late spring, summer and early autumn, but low during winter in the North-facing, permanently shaded, side and extremely low in the South-facing, exposed side. Corresponding mid-day PSII effective yield and linear electron transport rates peaked at late spring and early summer with the exposed side always displaying lower values for effective yield, but higher values for electron transport rate. However, corticular electron transport rates were more than sixfold lower compared to leaves. Non-photochemical quenching was higher in the exposed side throughout the year while peak values appeared at early autumn. Although a photoinhibitory damage during winter can be claimed, we may note that Mediterranean winter temperatures are mild, while the light reaching the trunk photosynthetic tissues is very low (maximum at 30 and 280 μmol m⁻² s⁻¹ in the shaded and the exposed side, respectively) to be considered as photoinhibitory. Based on recent findings for the retention of PSI activity and a concomitant inhibition of PSII under low temperatures in leaves, together with an adequate cyclic electron flow found in bark chlorenchyma, we suggest a temperature-dependent adaptive adjustment in the relative rates of PSI over PSII activity, possibly linked to seasonally changing needs for metabolic energy supply.     

The xanthophyll cycle and sustained thermal energy dissipation activity in Vinca minor and Euonymus kiautschovicus in winter

The influence of low temperature on the operation of the xanthophyll cycle and energy dissipation activity, as ascertained through measurements of chlorophyll fluorescence, was examined in two broad-leaved evergreen species, Vinca minor L. and Euonymus kiautschovicus Loessner. In leaves examined under laboratory conditions, energy dissipation activity developed more slowly at lower leaf temperatures, but the final, steady-state level of such activity was greater at lower temperatures where the rate of energy utilization (through photosynthetic electron transport) was much lower. The rate at which energy dissipation activity increased was similar to that of the de-epoxidation of violaxanthin to antheraxanthin and zea-xanthin at different temperatures. However, leaves in the field examined prior to sunrise on mornings following cold days and nights exhibited a retention of antheraxanthin and zeaxanthin that was associated with sustained decreases in photosystem II efficiency. We therefore suggest that this phenomenon of ‘photoinhibition’ in response to light and cold temperatures during the winter results from sustained photoprotective thermal energy dissipation associated with the xanthophyll cycle. Such retention of the de-epoxidized components of the xanthophyll cycle responded to day-to-day changes in temperature, being greatest on the coldest mornings (when photoprotective energy dissipation might be most required) and less on warmer mornings when photosynthesis could presumably proceed at higher rates.     

Contribution of central versus sweat gland mechanisms to the seasonal change of sweating function in young sedentary males and females

In summer and winter, young, sedentary male (N = 5) and female (N = 7) subjects were exposed to heat in a climate chamber in which ambient temperature (Ta) was raised continuously from 30 to 42°C at a rate of 0.1°C min⁻¹ at a relative humidity of 40%. Sweat rates (SR) were measured continuously on forearm, chest and forehead together with tympanic temperature (Tty), mean skin temperature ( [`T] s ) \left( {\overline {\hbox{T}} {\hbox{s}}} \right) and mean body temperature ( [`T] b ) \left( {\overline {\hbox{T}} {\hbox{b}}} \right) . The rate of sweat expulsions (Fsw) was obtained as an indicator of central sudomotor activity. Tty and ( [`T] b ) \left( {\overline {\hbox{T}} {\hbox{b}}} \right) were significantly lower during summer compared with winter in males; SR was not significantly different between summer and winter in males, but was significantly higher during summer in females; SR during winter was higher in males compared with females. The regression line relating Fsw to ( [`T] b ) \left( {\overline {\hbox{T}} {\hbox{b}}} \right) shifted significantly from winter to summer in males and females, but the magnitude of the shift was not significantly different between the two subject groups. The regression line relating SR to Fsw was steepened significantly from winter to summer in males and females, and the change in the slope was significantly greater in females than in males. Females showed a lower slope in winter and a similar slope in summer compared to males. It was concluded that sweating function was improved during summer mediated by central sudomotor and sweat gland mechanisms in males and females, and, although the change of sweat gland function from winter to summer was greater in females as compared with males, the level of increased sweat gland function during summer was similar between the two subject groups.     

Parallel evolution of thermophilia: daily and seasonal foraging patterns of heat‐adapted desert ants: Cataglyphis and Ocymyrmex species*

In the deserts of northern and southern Africa, respectively, ants of the genera Cataglyphisf oerster (Formicinae) and Ocymyrmexe mery (Myrmicinae) occupy the same ecological niche, which comprises that of a strictly diurnal thermophilic scavenger. Their daily foraging activities exhibit a bimodal pattern in summer and unimodality or complete inactivity in winter. The present study investigates whether these overall patterns are a result of endogenous annual activity rhythms of the colony or are triggered directly by the prevailing ambient temperatures. By exploiting various seasonal temperature regimes and, in particular, by creating near‐nest winter conditions experimentally in summer, it is shown that the latter hypothesis is generally true. However, there are daily and annual variations in the temperature set points at which foraging activities start and finish. These temperatures are lower in the winter than in the summer months and, in summer, they are lower in the morning than in the afternoon. The level of foraging activity in the afternoon reaches maximum values at surface temperatures of 60–63 °C. This means that, in summer months, these thermophilic ants concentrate their foraging activities into a period of almost lethal temperature regimes, during which they have to devote a substantial portion of their time outside the nest to respite (i.e. cooling‐off) behaviour.     

Sucrose metabolism in spring and winter wheat in response to high irradiance, cold stress and cold acclimation

Effects of low‐temperature stress, cold acclimation and growth at high irradiance in a spring (Triticum aestivum L. cv. Katepwa) and a winter wheat (Triticum aestivum L. cv. Monopol) were examined in leaves and crowns with respect to the sucrose utilisation and carbon allocation. Light‐saturated and carbon dioxide (CO₂)‐saturated rates of CO₂ assimilation were decreased by 50% in cold‐stressed spring and winter wheat cultivars. Cold‐ or high light‐acclimated Katepwa spring wheat maintained light‐saturated rates of CO₂ assimilation comparable to those of control spring wheat. In contrast, cold‐ or high light‐acclimated winter wheat maintained higher light and CO₂‐saturated rates of CO₂ assimilation than non‐acclimated controls. In leaves, during either cold stress, cold acclimation or acclimation to high irradiance, the sucrose/starch ratio increased by 5‐ to 10‐fold and neutral invertase activity increased by 2‐ to 2.5‐fold in both the spring and the winter wheat. In contrast, Monopol winter wheat, but not Katepwa spring wheat, exhibited a 3‐fold increase in leaf sucrose phosphate synthase (SPS) activity, a 4‐fold increase in sucrose:sucrose fructosyl transferase activity and a 6.6‐fold increase in acid invertase upon cold acclimation. Although leaves of cold‐stressed and high light‐grown spring and winter wheat showed 2.3‐ to 7‐fold higher sucrose levels than controls, these plants exhibited a limited capacity to adjust either sucrose phosphate synthase or sucrose synthase activity (SS[s]). In addition, the acclimation to high light resulted in a 23–31% lower starch abundance and no changes at the level of fructan accumulation in leaves of either winter or spring wheat when compared with controls. However, high light‐acclimated winter wheat exhibited a 1.8‐fold higher neutral invertase activity and high light‐acclimated spring wheat exhibited an induction of SS(d) activity when compared with controls. Crowns of Monopol showed higher fructan accumulation than Katepwa upon cold and high light acclimation. We suggest that the differential adjustment of CO₂‐saturated rates of CO₂ assimilation upon cold acclimation in Monopol winter wheat, as compared with Katepwa spring wheat, is associated with the increased capacity of Monopol for sucrose utilisation through the biosynthesis of fructans in the leaves and subsequent export to the crowns. In contrast, the differential adjustment of CO₂‐saturated rates of CO₂ assimilation upon high light acclimation of Monopol appears to be associated with both increased fructan and starch accumulation in the crowns.     

Gender differences in activity patterns of American mink <Emphasis Type="Italic">Neovison vison</Emphasis> in Germany

Circannual and circadian activity patterns of American mink Neovison vison were studied in a fishpond area in northern Germany. Fourteen mink were radiotracked, nine of them (five males, four females) for at least 6 months, some of them even up to 21 months. Both sexes have reduced activity rates in winter months and showed a considerable increase of activity during mating season (March). In summer months—during the pup raising period—female activity continued to be high. Male mink, on the other hand, were less active with the exception of August and September. During this time, dispersal of juveniles takes place with resident males increasingly having to defend their territories. Circadian activity rhythms differed markedly between sexes. All investigated females exhibited a perennial diurnal pattern. Female activity during twilight and night was significantly lower than during daylight. Three investigated male mink showed a typical nocturnal course throughout the year. Daylight activity of these animals was very low, increased during dusk and dawn activity rates and stayed at a higher level during the night. Two males behaved arrhythmic; they did not prefer any time of the day for activity. The observed temporal strategies were discussed in context of intra-sexual territoriality, sexual dimorphism in body size and different environmental factors.     

Feedback-limited photosynthesis and regulation of sucrose-starch accumulation during cold acclimation and low-temperature stress in a spring and winter wheat

Regulation of sucrose-starch accumulation and its effect on CO₂ gas exchange and electron transport were studied in low-temperature-stressed and cold-acclimated spring (Katepwa) and winter (Monopol) cultivars of wheat (Triticum aestivum L.). Low-temperature stress of either the spring or winter cultivar was associated with feedback-limited photosynthesis as indicated by a 50–60% reduction in CO₂ assimilation rates, twofold lower ATP/ADP ratio, and threefold lower electron transport rate than 20°C-grown control plants. However, no limitations were evident at the level of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) in low-temperature-stressed plants. Cold acclimation of the spring cultivar resulted in similar feedback-limited photosynthesis observed during low-temperature stress. In contrast, cold acclimation of the winter cultivar resulted in an adjustment of CO₂ assimilation rates to that of control plants. However, we show, for the first time, that this capacity to adjust CO₂ assimilation still appeared to be associated with limited triose phosphate utilisation, a twofold lower ATP/ADP ratio, a reduction in electron transport rates but no restriction at the level of Rubisco compared to controls grown at 20°C. Thus, contrary to previous suggestions, we conclude that cold-acclimated Monopol appears to exhibit feedback limitations at the level of electron transport characteristic of cold-stressed plants despite the maintenance of high rates of CO₂ assimilation. Furthermore, the differential capacity of the winter cultivar to adjust CO₂ assimilation rates was associated with higher levels of sucrose accumulation and a threefold higher sucrose-phosphate synthase activity despite an apparent limitation in triose phosphate utilisation.Abbreviations AGPase ADP-glucose pyrophosphorylase - FBPase fructose-1,6-bisphosphatase - Fru 6-P fructose 6-phosphate - Fru 1,6-BP fructose 1,6-bisphosphate - Glc 6-P glucose 6-phosphate - PGA 3-phosphoglyceric acid - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase - RuBP ribulose 1,5-bisphosphate - SPS sucrose-phosphate synthase - Triose-P triose phosphate     

Influence of environmental conditions on demand-feeding behaviour of gilthead seabream (Sparus aurata)

We studied the demand‐feeding behaviour was studied of gilthead seabream (Sparus aurata, L.) reared under either constant (25 ± 0.5°C, 12 : 12 L : D, control group) or natural (experimental group) temperature and photoperiod conditions during a period from winter to summer. Hourly demand‐feeding activity profiles were recorded using self‐feeding devices; these profiles showed that control group behaved entirely as a diurnal species, exhibiting no nightly activity and decreased demand rates in winter months. The experimental group did exhibit nightly activity (in incomplete darkness); this group also showed reduced demand rates in winter months, accompanied by a demand peak shift towards evening/night hours that followed the day's temperature peak of colder months.     

Seasonal variations in the rates of aquatic and aerial respiration and ammonium excretion of the ribbed mussel, Geukensia demissa (Dillwyn)

Aquatic and aerial rates of oxygen consumption and ammonium excretion of ribbed mussels, Geukensia demissa (Dillwyn), collected from the mid-intertidal zone of a mid-Atlantic salt marsh, were measured under ambient conditions of food, temperature, and salinity over five seasons. Rates of aquatic respiration covaried with body size and season, as the rates were high and strongly related to mussel tissue weight in spring and summer but low and weight independent in winter. There was a significant interannual difference between summer of 1995 and 1996. Rates of aerial respiration also varied seasonally, with high rates of oxygen consumption in summer and low rates in winter. The magnitude of these seasonal variations were greater than those for aquatic respiration, and as a result, the ratio of aerial to aquatic respiration was higher in summer (0.61 and 0.52) than in winter (0.11). This indicates that G. demissa was able to actively regulate aerial respiration, thereby permitting high aerobic metabolism during prolonged periods of air exposure in summer. We hypothesize that such high rates of aerial respiration, during the seasons of high metabolic activity, are required to provide sufficient energy for mussels to facilitate food digestion during air exposure at low tide. Rates of ammonium excretion varied seasonally and increased with mussel weight in all seasons. The atomic ratio of oxygen to nitrogen (O:N), calculated from aquatic respiration vs. ammonium excretion, was significantly lower in autumn (26) than in other seasons (46–60) among which the O:N did not vary significantly.